Multiphasic flows in hydrocarbon wells are very difficult to evaluate and measure as they can encounter complex flow regimes such as bubble, slug, mist or segregated flow depending on fluid properties, holdups, rates. Since hydrocarbon wells are generally deviated, sometimes highly deviated and even horizontal, the proportions of the phases (oil, gas, water) which make up the fluid flowing in the well are not homogeneous across a given section of the well and a mapping across the section is desirable.
In order to map the well section, local sensors are required for fluid identification, i.e. the evaluation of phase concentrations at each position of the well section.
The prior art discloses resistivity sensors for determining local phase concentrations in the fluid flowing in a hydrocarbon well.
The document U.S. Pat. No. 3,792,347 discloses the use of a plurality of needle-shaped sensing pointed electrodes. This document describes a tool for measuring the percent of oil in an oil/water mixture in an oil well. A plurality of electrodes is arranged in a substantially coextensive array in the flow path of the produced mixture at varying levels. Electrical arrangements are provided whereby a statistical analysis of the number of electrodes immersed in oil at a given moment may be integrated on a go-no go basis, the electrodes being either insulated when in oil or conducting to ground when in water. The number of electrodes so insulated by oil droplets being converted into an analogous electrical signal. Errors due to variations in the resistance and galvanic offset voltage in the water are compensated for.
The document U.S. Pat. No. 5,661,237 discloses a measurement sonde comprising a plurality of local resistivity probes. This document describes a method for producing, in a hydrocarbon well, a signal indicative of a local flow parameter of a multiphase fluid, includes the steps of placing at least one local sensor in the fluid and producing a signal whose level is characteristic of the phase in which the sensor is immersed, the signal being generated at a spike whose radius of curvature is less than 100 microns. The method is applicable to determining hold-ups of different phases of the fluid.
Electrical probes as disclosed in the document U.S. Pat. No. 5,661,237 have a coaxial structure. Such a probe according to the state of the art is depicted in FIGS. 1a, 1b, 1C and 1d. The probe 1 is composed of an external metal tube 2 and a central metal electrode 4 separated by an insulator 3. The tip 5 of the probe 1 is shaped into a cone in order to make a sharp end facilitating fluid 10 interface piercing. The measurement is made by measuring the impedance between the external electrode 2 (ground electrode) and the central electrode 4, which is related to fluid 10 resistivity, permittivity as well as probe geometry. FIG. 1a is an external view of such an electrical probe. The measurements enable detecting water in a multiphase fluid 10 flowing in hydrocarbon wells. FIG. 1b is a cross-section view of the tip 5 construction for such an electrical probe. FIG. 1c is a cross-section view showing current lines 6 extending in a large volume from the probe tip inducing large sensitivity to bubble/droplet sizes and continuous medium properties. FIG. 1d shows a probe response u(t) to a moving fluid interface for such an electrical probe (full line u1) and for an alternative electrical probe having an extended ground needle (dotted line u2). The moving fluid interface corresponds to the electrode penetrating an oil droplet in water. The probe response u(t) at various fluid interface Fla (before penetration), Flb (penetration at the end of the tip), Flc (half penetration of the tip) and Fld (full penetration of the tip).
The drawbacks of such a probe structure are the following:                The sensitivity extends far away from the probe tip leading to unstable measurements in multiphasic conditions. In particular the change of continuous medium from water to oil or gas has a large influence on measurements.        The measurement depends on electrode resistivity and contact resistance between metal and fluid where it is immersed. In high salinity brines the contribution of fluid resistivity to signal can become small leading to poor accuracy on measurements.        External electrical fields can generate parasitic currents in the external tube of the probe. In particular ground currents generated in the measurement sonde are difficult to isolate from the external tube of the probes and create noise in measurements.        Due to the above described limitations, it is not possible to accurately measure electrical properties of fluids. In practice such probes are used to discriminate water from oil and gas giving a digital-like signal. In particular it is not possible is to interpret water salinity.        Chemical and electromechanical effects can lead to the growth of an insulating layer at the metal surface degrading the mechanical integrity of the probes and altering measurements.        The time response is slow as transition requires a complete probe immersion in water up to the external tube metal contact in order to define a return path for the current.        The metal tip of the probe is facing the flow with maximum fluid velocity on its sharp end. In high flow conditions the sensitive element of the sensor is therefore submitted to erosion by solid particles such as sand grains embarked with flow leading to probe degradation in the field and alteration of signal over time.        Oil or water droplets can stick at the end of the probe, in particular at the insulator-metal interface.        